Signboard is a tiny Android application to display given message as big as device's screen can hold.
It can be used in 3 different ways:

direct mode

Run the application by clicking on its icon .
Long touch of the screen allows to edit the text to show.

After completion, Signboard still remembers the text and will show it again on the next run.

widget mode

You can add a widget to the home screen by long press on an empty spot.
Next, select Signboard from the list of widgets, and drag it to the home screen with the long press.
For complete widget creation, it is necessary to define the text to be displayed.
The icon of the widget takes an image of the text to display.

You can have many widgets, each one with its own text.
The text can be updated with long press on the display (not the icon).

slave/intent mode

In this mode, Signboard can be launched by other application capable of sending text with the share, send, etc.
There are plenty of such applications. For instance I chose default web browser, available on Samsung phones.

from the certain page, I've selected a piece of text

when the menu popped up, I chose Share

then I chose Signboard from the list of applications

Since the text is handled by other application, there is no possibility to edit nor save it.
Instead, you can steal it to the clipboard with the long press.